Method of improving machinability of steel



United States Patent METHOD OF IMPROVING MACHINABILITY or STEEL itRobert W. Baughman, Youngstown, Ohio, and Raymond A. Grange, Livingston,N. J., assignors to United States Steel Corporation, a corporation ofNew Jersey No Drawing. Application November 4, 1955,

Serial No. 545,112

2 Claims. (Cl. 148-215) This invention relates to a method of improvingthe machinability of steel and more particularly to improving themachinability of carburizing grades of steel.

Many machine parts, such as gears, are made of low carbon steelcontaining less than 30% carbon and are carburized to produce a hardwear resistant surface with a tough low carbon core. The parts areordinarily machined to final size or to approximate size prior tocarburizing and afterwards finished to final size by grinding orlapping. Since the finished parts must be quite smooth, the smoothnessof the surface following machining is of great importance since itdetermines the amount of expensive grinding or lapping required toachieve the de sired degree of smoothness.

Heretofore it has been common practice to normalize or normalize andtemper these steels prior to machining. This frequently causes a gummycondition which results in a rough machined surface because the steel isquite soft and is striated, i. e. contains bands of ferrite.

We have discovered that the rough machined surface resulting from theforegoing treatment can be eliminated, and a smooth unpitted surfaceobtained by treating ferritic steel containing less than 3% carbon, withthe ,balance iron and other elements in amounts which do not adverselyaffect the properties in the following manner prior to machining. Suchsteel in the as-rolled or as-forged condition is heated or austenitizedat a temperature determined by the formula when TA is the furnacetemperature, Ael the lower equilibrium temperature and Airs the upperequilibrium temperature. The material should be held in the furnaceuntil the temperature of the workpiece is equilized throughout atsuchtemperature. By using the temperature derived by the formula, theworkpiece will have a structure of about.50% austenite -50% ferrite,110%. This structure is essential since if less austenite than'this isformed the final structure will be too soft and machine in a gummyfashion; if more austenite than this forms, the final structure will betoo hard. After the temperature has been equalized at the TA holdingtemperature, the workpiece may be cooled in any convenient way to thebainite formation range, i. e. about 500 to 1000 F. for steelscontaining up to 3% carbon. The steel must be coo-led slowly enoughthrough this range to insure essentially complete transformation tobainite. In some steels, depending on alloy content, an air cool willsuffice,

in others retarded cooling will be necessary. Such retarded cooling maybe accomplished by moving the workpiece to a cooler section of thefurnace, moving to a second furnace or by turning the furnace down toabout 500 F, i. e. about the M5 point of the untransformed austenite andpermitting the temperature of the workpiece to equalize thereat and holdfor any additional time as may be required to complete thetransformation to temperature in any convenient manner.

2,779,698 Patented J 29, .1957- A sample of the steel so treated wasquenched to room temperature and had a microstructure of about 50%ferrite and 50% austenite. Following this, the bars were cooled in thefurnace from 1410 to 500 F. and then removed and cooled in air to roomtemperature. During cooling from 1410 to about 1000 F., about half ofthe austenite formed at 1410" F. transformed to additional ferriteleaving comparatively small uniformly distributed volumes of austenite.The latter are greatly enriched in carbon by the rejection of ferrite,that is, most of the carbon of the steel is present in the austenitephase. On cooling from 1000 to 500 F. most of the austenite transformedto bainite leaving small, uniformly distributed colonies of bainite. Thefew small particles of what was either austenite or martensite whichremained were too few and too small to appreciably influence machining.

Bars of SAE 4317 steel of the same composition as the foregoing weretreated in the conventional way by normalizing from 1700 F. and thentempering at 1200 F. These bars had a severely banded structure and weremuch rougher after machining. i

It was also observed that the bars treated in accordance with thisinvention had more broken-up chips, especially, on the lighter cuts, anddid not become hot indicating that less energy was required. They alsosawed in about /6 less time than the normalized and tempered bar.

While we have described one specific embodiment of our invention, itwill be understood that this embodiment is merely for the purpose ofillustration and description and that various other forms may be devisedwithin the scope of our invention, as defined in the appended claims.

We claim:

1. A method of improving the machinability of ferritic steels containingless than 3% carbon with the balance iron and other elements in amountswhich do not adversely affect the properties, comprising heating saidsteel at a temperature determined by the formula TA ==Ae1+% (Ass-A21)wherein TA is the furnace temperature Aei is the lower equilibriumtemperature Aes is the upperequilibrium temperature to partiallyaustenitize it, cooling said steel to the bainite transformation range,cooling through'such range at a sufliciently slow rate to insureessentially complete transformation of the austenite to bainite andthereafter cool- TA =A1-l-%(A3Al) wherein i TA is the furnacetemperature Aer is the lower equilibrium temperature A23 is the upperequilibrium temperature for sufiicient time to equalize the temperatureof the steel References Cited in the tile of this patent at said TAtemperature to produce a structure therein UNITED STATES PATENTS whichis substantially 50% austenite and 50% ferrite, cooling said steel tobelow about 1000 F. and cooling grabs 1 said steel through its bainitetransformation range at a 5 2664369 122: 1953 sufiiciently slow rate toinsure essentially complete transformation of the austenite to bainiteand thereafter cool- OTHER REFERENCES ing it to room temperature. MetalProgress, October 1949, pages 504-505.

1. A METHOD OF IMPROVING THE MACHINABILITY OF FERRITIC STELLS CONTAININGLESS THAN .3% CARBON WITH THE BALANCE IRON AND OTHER ELEMENTS IN AMOUNTSWHICH DO NOT ADVERSELY AFFECT THE PROPERTIES, COMPRISING HEATING SAIDSTEEL AT A TEMPERATURE DETERMINED BY THE FORMULA